Engine combustion chamber valves, such as intake and exhaust valves, are typically spring biased toward a valve closed position. In many internal combustion engines, the engine valves may be opened and closed by fixed profile cams in the engine. More specifically, valves may be opened or closed by one or more fixed lobes which may be an integral part of each of the cams. In some cases, the use of fixed profile cams may make it difficult to adjust the timings and/or amounts of engine valve lift. It may be desirable, however, to adjust valve opening times and lift for various engine operating conditions, such as different engine speeds.
A method of adjusting valve timing and lift, given a fixed cam profile, has been to incorporate a “lost motion” device in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion dictated by a cam profile with a variable length mechanical, hydraulic, or other linkage means. The lost motion system comprises a variable length device included in the valve train linkage between the cam and the engine valve. The lobe(s) on the cam may provide the “maximum” (longest dwell and greatest lift) motion needed for a range of engine operating conditions. When expanded fully, the variable length device (or lost motion system) may transmit all of the cam motion to the valve, and when contracted fully, transmit none or a reduced amount of cam motion to the valve. By selectively decreasing the length of the lost motion system, part or all of the motion imparted by the cam to the valve can be effectively subtracted or lost.
Hydraulic-based lost motion systems may provide a variable length device through use of a hydraulically extendable and retractable piston assembly. The length of the device is shortened when the piston is retracted into its hydraulic chamber, and the length of the device is increased when the piston is extended out of the hydraulic chamber. One or more hydraulic fluid control valves may be used to control the flow of hydraulic fluid into and out of the hydraulic chamber.
One type of lost motion system, known as a Variable Valve Actuation (VVA) system, may provide multiple levels of lost motion. Hydraulic VVA systems may employ a high-speed control valve to rapidly change the amount of hydraulic fluid in the chamber housing the hydraulic lost motion piston. The control valve may also be capable of providing more than two levels of hydraulic fluid in the chamber, thereby allowing the lost motion system to attain multiple lengths and provide variable levels of valve actuation.
Typically, engine valves are required to open and close very quickly, and therefore the valve return springs are generally relatively stiff. If left unchecked after a valve opening event, the valve return spring could cause the valve to impact its seat with sufficient force to cause damage to the valve and/or its seat. In valve actuation systems that use a valve lifter to follow a cam profile, the cam profile provides built-in valve closing velocity control. The cam profile may be formed so that the actuation lobe merges gently with cam base circle, which acts to decelerate the engine valve as it approaches its seat.
In hydraulic lost motion systems, and in particular VVA hydraulic lost motion systems, rapid draining of fluid from the hydraulic circuit may prevent the valve from experiencing the valve seating provided by cam profile. In VVA systems, for example, an engine valve may be closed at an earlier time than that provided by the cam profile by rapidly releasing hydraulic fluid from the lost motion system. When fluid is released from the lost motion system, the valve return spring may cause the engine valve to “free fall” and impact the valve seat at an unacceptably high velocity. The valve may impact the valve seat with such force that it eventually erodes the valve or valve seat, or even cracks or breaks the valve. In such instances, engine valve seating control may be desired because the closing velocity of the valve is governed by the release of hydraulic fluid from the lost motion system instead of by a fixed cam profile. Accordingly, there is a need for valve seating devices in engines that include lost motion systems, and most notably in VVA lost motion systems.
In order to avoid a damaging impact between the engine valve and its seat, the valve seating device should oppose the closing motion regardless of the position of other valve train elements. In order to achieve this goal, the point at which the engine valve experiences valve seating control should be relatively constant. In other words, the point during the travel of the engine valve at which the valve seating device actively opposes the closing motion of the valve should be relatively constant for all engine operating conditions. Accordingly, it may be advantageous to position the valve seating device such that it can oppose the closing motion of the engine valve without regard to the position of intervening valve train elements, such as rocker arms, push tubes, or the like.
The valve seating device may include hydraulic elements, and thus may need to be supported in a housing and require a supply of hydraulic fluid, yet at the same time fit within the packaging limits of a particular engine. It may also be advantageous to locate the valve seating device near other hydraulic lost motion components. By locating the valve seating device near other lost motion components, housings, hydraulic feeds, and/or accumulators may be shared, thereby reducing bulk and the number of required components.
A valve seating device may be constructed so that a significant portion of the opposing force it applies to a closing engine valve occurs during the last millimeter of travel of the valve. As a result, control of the amount of lash space between the valve seating device and the engine valve or other intervening elements may be critical to proper operation of the valve seating device. Factors such as component thermal growth, valve wear, valve seat wear, and tolerance stack-up can affect the amount of lash. Some known valve seating devices have required manual lash adjustment or a separate set of lash adjustment hardware. Accordingly, it may be advantageous to have a valve seating device that self-adjusts for lash differences between the engine valve and the valve seating device.
Various embodiments of the present invention may meet one or more of the aforementioned needs and provide other benefits as well.